Recent years, as a clean energy source capable of highly efficient energy conversion and friendly to the global environment, a fuel cell has attracted an attention. Among a variety of the fuel cells, a solid oxide fuel cell is the one, in which oxide ion conductive solid electrolyte such as yttria stabilized zirconia is used as electrolyte, and porous electrodes are attached onto both surfaces (back and front surfaces) thereof. The solid oxide fuel cell is a cell in a form, in which the solid electrolyte is used as a partition, fuel gas such as hydrogen and hydrocarbon is supplied to a surface of the partition on one electrode side, and air or oxygen gas is supplied to the other surface thereof on the other electrode side. In general, the solid oxide fuel cell is operated at a high temperature of about 1000° C. or higher.
It is known that conductivity of the solid electrolyte is about one-tenth conductivity of electrolyte of a phosphoric acid fuel cell or a fused carbonate fuel cell. Generally, since electric resistance of the electrolyte portion becomes a loss of power generation, it is important to reduce film resistance as much as possible by thinning a film of the solid electrolyte in order to enhance a power generation output density. However, since an area of a certain extent or more is required for the electrolyte portion in order to secure a function as a cell, for the solid oxide fuel cell, a cell structure (single cell structure) is adopted, in which a solid electrolyte film is formed on a support having mechanical strength. As specific structures of the solid oxide fuel cell, structures as below are proposed.
(1) Cylinder Type SOFC
A cylinder type is the one, in which a cylindrical and porous base tube is used as a support, and a cell structure having a fuel electrode layer, an electrolyte layer and an air electrode layer stacked therein is formed on a surface of the base tube.
There are a cylindrical cross-stripe type in which a plurality of single cell structures are arrayed in one base tube and a cylindrical pinstripe type in which one single cell is formed in one base tube. In any of the types, a plurality of cylinders are electrically connected from one to another by interconnectors to constitute a cell, any one of the fuel gas or the air is introduced in the inside of the base tube, and the other thereof is introduced in the outside of the base tube, thus generating power. In the cylinder type solid oxide fuel cell as described above, since one of the fuel gas and the air is flown into the base tube, there is a feature in that a seal is not particularly required between the fuel gas and the air.
(2) Flat Plate Type SOFC
A flat plate type basically has a structure equivalent to that of the phosphoric acid or fused carbonate fuel cell. Specifically, the flat plate type has a structure, in which separator plates and plane-shaped cell plates are alternately adhered. Each separator plate has fuel electrode plates where fuel gas passages are formed and air electrode plates where air passages are formed, which are adhered to both surfaces of interconnector flat plates; and each plane-shaped cell plate has fuel electrode layers and air electrode layers stacked on both surfaces of a sheet-shaped electrolyte layer.
In order to thin the electrolyte layer, a cell structure is proposed, in which any one electrode layer of the porous fuel electrode and air electrode is taken as a support, and the electrolyte film and the other electrode layer are formed thereon. For example, a constitution is disclosed, in which an electrolyte layer having a film thickness of 15 μm is formed on a fuel electrode layer made of Ni cermet having a thickness of 1.5 mm by a vacuum slip casting method (Proceedings of The 3rd International Fuel Cell Conference, P349).
(3) Monolith Type
The monolith type has a structure analogous to that of the flat plate type. The monolith type has a structure, in which separator plates having fuel electrode layers where the gas passages are not formed and the air electrode layers formed on the both surfaces of the interconnector flat plates, and a three-layered structure of cell films having waveplate-shaped fuel electrode layers, electrolyte layers and air electrode layers, are alternately adhered. There is a feature in that the passages are formed by use of the waveform shape of the cell film and the area of the electrolyte is enlarged, thus resistance of the electrolyte film is reduced.
(4) Thin Film Type
As a structure of a fuel cell having a film thickness of the electrolyte further thinned (herein, referred to as a “thin film type”), a structure is proposed, in which a large number of micro-openings are formed on a substrate, and cell plates and the separator plates having the passages formed thereon are alternately stacked, each cell plate being constituted by adhering three-layered films of the fuel electrode layer, the electrolyte layer and the air electrode layer onto these micro-openings (Japanese Patent Laid-Open Publication No. H8-64216 (published in 1996). It is described that, with this structure, a nonporous silicon (Si) wafer is used as a support substrate, and films are formed thereon, thus making it possible to set the film thickness of the electrolyte at about 2 μm. Specifically, on either a Si substrate or an oriented cerium oxide (CeO2) layer formed on the Si layer, an electrolyte layer made of stabilized zirconia in a single crystal film is formed.
Moreover, similarly, a cell structure is proposed, in which micro-openings are formed on a single crystal Si substrate insulated and coated with a silicon nitride film, and the three-layered films of the fuel electrode layers, the electrolyte layers and the air electrode layers are formed on these openings (Mat. Res. Soc. Symp. Proc. Vol. 496, p155).